By Dan Kay
|A 304-stainless bar-stock component that was vacuum-brazed with AMS 4786 brazing filler metal (BFM), for use in ground-power turbines. Photo courtesy of Woodward Fuel Systems (Greenville, SC)|
Brazing has proven to be a highly versatile joining process for permanently joining many different kinds of metals and ceramic materials together in a variety of industries as diverse as aerospace and automotive.
Brazing has a long history of use, dating back to the time of the ancient Egyptians, and truly came into its own as a high-volume production technique during the twentieth-century, not only via torch-brazing, but especially with the development of well-controlled continuous and batch-type brazing furnaces. In the last few decades, the steady increase in vacuum furnace technology made vacuum brazing a preferred method by the end of the twentieth century for many companies doing brazing.
To use equipment effectively for brazing, you must first understand what brazing is, and what is required to make a good braze. You need to know how brazing compares to welding and soldering. In this and future articles, I will talk about the fundamentals of brazing and how to apply them, so that brazing companies can prevent many of the common problems that brazers face, and thereby increase their production rates and quality as well as lower their overall costs.
So, what then is "brazing"?
Brazing is a process that permanently joins two or more metals/materials together to form a single assembly by heating them in the presence of a filler metal that begins to melt above 450°C (840°F). The liquid filler metal is drawn into the gap between the closely fitted faying surfaces of the joint by capillary action.
|Fig. 1. Faying Surfaces (the important surfaces for brazing)|
Thus, in brazing, the base materials (metals and/or ceramics) are not melted. Only the brazing filler metal (BFM) becomes liquid, and flows by capillary action between the surfaces of the non-melted base materials, as shown in Fig. 1. These surfaces of interest here are called the "faying surfaces", that is, the surfaces inside the joint that are to be joined together.
Surfaces outside of the joint are generally irrelevant. The same holds true for the fillets formed at the outside edge of the joint by the BFM---they don't really contribute to the strength of the joint. Thus, only what happens INSIDE the joint, i.e., between the faying surfaces, matters. And that, in turn, depends on the ability of capillary action to do its work. Therefore, it is very important to understand what variables will enhance or hinder its work.
|Fig. 2 Wetting of base material|
Capillary action is the force that causes liquids to rise in a straw sitting in a glass, or what causes spilled coffee to be absorbed by a paper napkin. Capillary force is very strong---stronger than gravity inside a properly designed joint --- thus enabling a liquid brazing filler metal (BFM) to be drawn between the faying surfaces of the joint in any direction, irrespective of the orientation of the assembly (horizontal or vertical)For capillary force to work, however, the base metal and the BFM must be compatible, that is, they must be able to alloy with (diffuse into) each other. The BFM will then be able to "wet" (spread out along the surface of) the base metal. The extent of alloying doesn't have to be much, but it must occur.
Nickel and chromium, however, are able to alloy nicely with iron, and thus, the nickel-based BFMs are able to "wet" steel well, resulting in low wetting angles, as shown in Fig. 2 (top.) The smaller the wetting angle, the better. Pure silver, on the other hand, doesn't alloy well with pure iron. So when pure silver is melted on the surface of a steel (iron alloy), it tends to ball up, as illustrated in Fig. 2 (bottom), showing non-wetting of surfaces. It does not "wet" the steel, and has a large (poor) "wetting" angle (greater than 90°).
In my next article, I'll talk about vacuum atmospheres in brazing. Yes, vacuum is still an "atmosphere" in the technical sense of the word, since the gas inside the vacuum chamber has actually not been completely removed from the chamber, and those remaining molecules of gas must not be able to interact in a negative way with the faying surfaces of the brazement.
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